lkml.org 
[lkml]   [2024]   [Feb]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Subject[PATCH v2 03/14] crypto: ecdsa - Extend res.x mod n calculation for NIST P521
    Date
    res.x has been calculated by ecc_point_mult_shamir, which uses
    'mod curve_prime'. The curve_prime 'p' is typically larger than the
    curve_order 'n' and therefore it is possible that p > res.x >= n.

    If res.x >= n then res.x mod n can be calculated by iteratively sub-
    tracting n from res.x until n > res.x. For NIST P192/256/384 this can be
    done in a single subtraction. This can also be done in a single
    subtraction for NIST P521.

    The mathematical reason why a single subtraction is sufficient is
    due to the values of 'p' and 'n' of the NIST curves where the following
    holds true:

    note: max(res.x) = p - 1

    max(res.x) - n < n
    p - 1 - n < n
    p - 1 < 2n => true for the NIST curves

    Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
    ---
    crypto/ecdsa.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
    index 64e1e69d53ba..1814f009f971 100644
    --- a/crypto/ecdsa.c
    +++ b/crypto/ecdsa.c
    @@ -122,7 +122,7 @@ static int _ecdsa_verify(struct ecc_ctx *ctx, const u64 *hash, const u64 *r, con

    /* res.x = res.x mod n (if res.x > order) */
    if (unlikely(vli_cmp(res.x, curve->n, ndigits) == 1))
    - /* faster alternative for NIST p384, p256 & p192 */
    + /* faster alternative for NIST p521, p384, p256 & p192 */
    vli_sub(res.x, res.x, curve->n, ndigits);

    if (!vli_cmp(res.x, r, ndigits))
    --
    2.43.0

    \
     
     \ /
      Last update: 2024-05-27 15:05    [W:2.429 / U:0.052 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site